1. Field of the Invention
The present invention relates to an illuminant module and a method of preparing a liquid crystal display device and the illuminant module. More particularly, the present invention is concerned with a backlight that uses a light guide plate to illuminate a liquid crystal panel from the back of the liquid crystal panel.
2. Description of the Related Art
At present, light-emitting diodes (LEDs) provide only a lower luminous efficacy than a cold-cathode fluorescent lamp. Therefore, the LEDs are mounted at a high density, and a large current is fed to the LEDs in order to increase a luminous quantity. Moreover, the use efficiency of light is improved in order to draw out a luminance level equal to a luminance provided by the cold-cathode fluorescent lamp. When a large current is fed to the LEDs, the LEDs dissipate heat proportionally to the fed current. A rise in temperature derived from the heat dissipation deteriorates the performances of the LEDs themselves and peripheral devices respectively and adversely affects the long-term reliability of an illuminant module. As described in Patent Document 1, a structure having LED chips directly mounted on a metal substrate for the purpose of cooling the LEDs has been devised. The cooling structure successfully suppresses the rise in the temperature of the LEDs because the metal substrate absorbs or releases heat generated by the LEDs. However, the cooling structure makes it necessary to cover the entire LED chips and part of the surface of the metal substrate using a transparent encapsulating resin so as to immobilize the LED chips. The coefficient of thermal expansion of the transparent encapsulating resin to be used to encapsulate the LED chips is several times larger than that of a metallic material made into the metal substrate. Therefore, the rise in temperature occurring when the LEDs perform self-heating causes a difference in a magnitude of thermal expansion on an interface between the encapsulating resin and metal substrate that are bonded to each other. This may lead to bending or interfacial debonding of the bonded interface. In the structure having a plurality of LEDs encapsulated as a whole, the LEDs are divided into some blocks and then encapsulated in order to form several encapsulated blocks.
Patent Document 1: Japanese Patent Laid-Open No. 2002-299697
Formation of an LED illuminant module suitable for thin liquid crystal display devices requires a structure having LED chips directly mounted on a metal substrate like the one described in Patent Document 1. However, when a transparent encapsulating resin and the metal substrate are bonded to have a wide interface, a large thermal stress may be applied to each of the encapsulating resin and metal substrate. This may cause debonding of the bonded interface between the encapsulating resin and metal substrate or bending. When LEDs are divided into several blocks and encapsulated as they are conventionally, the seam between adjoining blocks may cause a luminance line. Eventually, homogeneous planar illumination may not be attained.